What comes to mind are things that I have been able to do with my own kids while juggling various ages & levels of need.
The younger one is still likely small enough to fit a toddler carrier. You can check, but many go to 50lbs or even a little more. It allows you (if the younger is a handful when walking ) to wear them on your back & most kids of that age seem to like that as long as you keep moving.
I don't know what season you are in or the weather, but water activities are always fun for all my kids at any age. It is a little harder when you may have two needing life vests, but you can make it work with some preparation & making all safety rules clear to the 6 year old before. Splash pads can be great, or a nightmare. They are nice that drowning is no concern, but if you have two that run, it's tough. I find a beach more challenging in general but they tend to be quite busy here & invariably all my kids want to go in different directions & hard for me to keep eyes on everyone.
There are some games that would be suitable. There is a game, called Doggy Doo, that is a favorite here for kids of almost any age. It's gross & silly & generally the rules or winning don't matter as much as the silliness. There are others like that, one that hits you in the face with whipped cream. I can't recall the name. Silly games are often fine, even with one more in a 2yr old level. They aren't meant for skill, they are all about the laughs.
And as far as handling the transition, it's a balance. I do think it's always good to allow your kids to feel their feelings. I also would encourage him to share his feelings while you help him frame it, understand it, find the vocabulary to share them. I also think it's important then to help him reframe it. He will need guidance in seeing how this could be a great thing & help to foster bonding between them. I have done this lots of ways. One of them is to buy something for the younger one I know the older one will show interest in, even if it's advanced for the younger one. So often the younger has nothing the older child wants. Your older one is still too young to notice that it might be a tactic and that you did so on purpose. It allows the younger child to have some currency with the older one, to be included, or permitted to play with something they would like, etc. I also pretend the non verbal child is telling me awesome things about the older one & tell the older one what they are saying. I have yet to meet a kid that doesn't love this. They do seem to know you are making it up and yet it still seems to build goodwill between the kids. We also play the "3 things I like about you game" and it really is exactly what it sounds like. We each have to say three things we like about someone else. The first person to get stumped is out. The winner never runs out of compliments. It can be anything from I like your brown hair to you make great paper airplanes to you are very smart, silly, strong, etc. You may have to speak for the younger child, but we always include them anyway, as you want your older children naming things they like about the little ones.
I hope any of that is of help. Changes are often hard, but I am sure with some time & patience this will all sort out. Congratulations on getting visitation as you wanted with your younger child.